Workaround for Crystal compiler bug

See: https://github.com/crystal-lang/crystal/issues/4225
If there are no concrete implementations of a class, an "undefined
method" error is given for abstract methods.

src/spectator/dummy_example.cr

@@ -0,0 +1,22 @@
+require "./runnable_example"
+
+module Spectator
+  # Example that does nothing.
+  # This is to workaround a Crystal compiler bug.
+  # See: [Issue 4225](https://github.com/crystal-lang/crystal/issues/4225)
+  # If there are no concrete implementations of an abstract class,
+  # the compiler gives an error.
+  # The error indicates an abstract method is undefined.
+  # This class shouldn't be used, it's just to trick the compiler.
+  private class DummyExample < RunnableExample
+    # Dummy description.
+    def what
+      "DUMMY"
+    end
+
+    # Dummy run that does nothing.
+    def run_instance
+      # ...
+    end
+  end
+end

src/spectator/includes.cr

@@ -16,6 +16,7 @@ require "./example_component"
 require "./example"
 require "./runnable_example"
 require "./pending_example"
+require "./dummy_example"
 
 require "./example_hooks"
 require "./example_group"

src/spectator/runnable_example.cr

@@ -4,7 +4,7 @@ module Spectator
   # Common base for all examples that can be run.
   # This class includes all the logic for running example hooks,
   # the example code, and capturing a result.
-  # Sub-classes need to implement the `#run_instance` method.
+  # Sub-classes need to implement the `#what` and `#run_instance` methods.
   abstract class RunnableExample < Example
     # Runs the example, hooks, and captures the result
     # and translates to a usable result.